1. Field of the Invention
The present invention relates to a method for adjusting the static magnetic field in a magnetic resonance imaging (MRI) device and a static magnetic field generating apparatus that generates a higher field strength. The present invention can be used In a C-type magnet of a magnetic resonance imaging apparatus, but is also suitable for use in other devices using an electra-magnet to generate a uniform static magnetic field, without limitation to the exemplified C-type.
2. Description of the Prior Art
In the early development of magnetic resonance imaging equipment for commercial use, the field strength of the permanent magnet was usually lower than 0.25 T. In the 1990's, magnetic resonance imaging equipment for commercial use were developed that employed a permanent magnet with a field strength of 0.3 T and the performance thereof was better, the cost was reasonable and the structure was compact. The greater the field strength, the higher the signal-to-noise ratio, and the better the quality of the image generated by the permanent magnet magnetic resonance equipment. In recent years, the field strength of some of the permanent magnets can reach 0.35 T-0.4 T.
At present, certain problems exist In the manufacturing of a magnet with a higher field strength:    I. Permanent magnets having a field strength greater than 0.3 T and with high homogeneous magnetic field in the scanning area are expensive, large in volume and heavy in weight (heavier than 17 tons) and need a large area for installation, and thus have limited application in magnetic resonance equipment.    II. Since a uniform strong magnetic field is to be generated, the size of the pole plates usually is large, thus the open angle between the two pole plates will inevitably be limited and this is a problem for patients subject to claustrophobia.    III. The magnet needs shimming or repairing during its installation and use, or re-shimming is required when service and thus the gradient coil and transmitting coil have to be removed, which is time-consuming and expensive.    IV. Due to manufacturing tolerances, magnets of the same design may have different field strengths, thus volume production of the same RF coils and the systems becomes very difficult and the costs for the corresponding system of the whole magnetic resonance equipment increase.
To address the above problems, the following methods are conventionally employed:    I. To increase the field strength and improve the field homogeneity, but this increases the volume and size of both the permanent magnet and the pole plate, which will increase the cost as well as the volume and weight of the magnet.    II. The homogeneity of the magnetic field is improved by employing a Rose ring but when the field strength is over 0.3 T, the magnetic field homogeneity of the permanent obtaining a magnetic resonance image of the whole human body becomes worse and a single Rose ring cannot ensure the homogeneity of the field.    III. Permanent magnets that reduce magnetic flux leakage are employed to compensate for the field strength leakage in the external edge of the pole plate, but this increases the size of the external edge of the magnet pole plate and reduces the open angle between the pole plates and furthermore requires complicated manufacturing process and hence increases the cost.    IV. Field strength adjusting and fine adjusting structures are employed, but these mechanisms are complex in design and expensive.
Moreover, the effects achieved by all of these known methods are still not satisfactory.